Grab a strand from each pair and clip a locking biner through all of them.

So, just in case, clip a locking biner from the skinny rope onto the good strand.

Take another sling or cord and tie a friction-hitch onto the loaded, climber's strand of the rope; clip a locking carabiner to it.

With the slack, clip a locking carabiner in front of the belay device and clip the climber's strand into it; lock the carabiner.

Clip a locking carabiner on your belay loop and hitch the cordelette to this locker using a Münter hitch.

Clip a locking carabiner through the rope, keeper loop on the device, and the belay loop on your harness.

Clip a locking biner to the rappel anchor and then clip the cordelette through that carabiner and lock it.